Plastic triplet objective lens system

ABSTRACT

A three element f/5.6 objective lens system is adapted for use with still picture cameras. All three elements of the system are preferably constructed of plastic materials.

United Sta D Grey v 5] Jan. 8, 1974 [5 PLASTIC TRIPLET OBJECTIVE LENS3,578,847 5/1971 Grey .3 350/226 SYSTEM [75] Inventor: David S. Grey,Waltham, Mass.

Primary Examiner-John K. Corbin [73] Assignee: Berkey Photo, Inc.,Clifton, NJ. Attorney sidney Neuman et 1 [22] Filed: Dec. 13, 1972 [21]Appl. No.: 314,724

[57] ABSTRACT [52] US. Cl 350/226, 350/175 NG, 350/206, 350/252 511 int.Cl. G02b 1/04, 0021) 9/16 A three element f/5-6 Objective lens System isadapwd 58 Field of Search 350/226 for use with still Picmre camerasthreeelements of the system are preferably constructed of plastic ma- [5 6]References Cited tenals- UNITED STATES PATENTS 3.438697 4/l969 Melech350/226 6 Claims, 2 Drawing Figures BACKGROUND OF THE INVENTION Thisinvention relates to an improved triplet objective lens systemparticularly adapted for use in connection with still picture cameras.

The present invention finds particular application in the low pricedphotographic cameras which are fabricated by means of high volumeproduction techniques and employ simple and inexpensive objectivelenses. It has been found that the cost of producing a triplet objectivelens system can be materially reduced ifthe lens elements of the systemcan be constructed entirely of plastic materials. Such plastic materialsnot only have the advantage over glass of low cost but they alsosimplify the manufacture of the system.

A lens system constructed in accordance with the present inventionpermits the individual lens elements to be molded out of plastic andprovides a system having a speed of f/5.6. 'I:he lens system, which isadapted for low cost fixed focus cameras, has goodoptical properties andmay be easily and inexpensively manufactured.

SUMMARY OF THE INVENTION In accordance with one aspect of the invention,a triplet objective lens system is provided consisting of two outerpositive lens elements and an intermediate negative lens element. Thefirst of the two outer positive lens elements is a frontal positivemeniscus lens, and the other of the positive lenses is a rearwardcondensing lens element. The lens system possesses the features,properties and relationships of the elements which will be exemplifiedin the detailed disclosure to follow. This system is so designed thateach of the three lens elements may be constructed of plastic materials,particularly acryIics'The indices of refractions permit the frontal lensto be formed of methyl methacrylate and for the other two lenses to beformed of polyacrylonitrile.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an axial cross sectionalview of a mounted triplet objective lens system constructed inaccordance with this invention; and

FIG. 2 is a tabular compilation of numerical constructional data for thepreferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. I, a lens mountI isillustrated for supporting the three lens elements I, II and III ofthe triplet objective lens system 12, in appropriately spaced, axialalignment. This particular lens mount is shown for illustration purposesonly, and this is but one type of mount which may be used with the lenssystem disclosed. Since the lens system is primarily intended for use inconnection with relatively low priced photographic cameras which arefabricated on a high volume production basis, it is preferred that thelens mount also be constructed so that its manufacture and the assemblyof the component lens elements therein are both adapted to massproduction techniques.

In this particular embodiment, the lens element III is mounted in acylindrical recess A at the rear of the lens mount 10, and theperipheral edge of this lens engages a forwardly facing shoulder 108.The other lens elements I and II are mounted in front of lens elementIII on a second cylindrical interior surface 100, and they may there beheld in place by suitable securing means such as a threaded rim or thelike (not shown). The lens elements are preferably so constructed thattheir nonoptical peripheral edge portions are in contact with oneanother, thus providing the desired accuracy in the spacing between thecentral optical portions of the lens elements and eliminating therequirement for special spacing devices within the lens mount 10. Spacedrearwardly of the lens mount 10 is a suitable stop or diaphragm I4, theposition of which is also set forth in Table I.

The three lens elements I, II and III of the triplet objective lenssystem 12 are spaced, simple lenses. Lens element I is a frontal,positive, biconvex meniscus lens; lens element II is a negative concavedispersing lens; and lens element III is a positive biconvex, rearwardcondensing lens.

Constructional values for the preferred embodiment of the invention areas shown in FIG. 2 and repeated in Table I below, in which the lenselements are numbered from front to rear. N is the index of refractionfor the D line of the spectrum, R is the radius of curvature of the lenssurface, T is the axial thickness of the lens element, and S is theaxial air spacing between the lens elements. The subscripts denotesurfaces, elements and air spaces numbered consecutively from front torear, and the and signs in the third column correspond to surfaces whichare respectively convex and concave to the front.

TABLE I Equivalent Focal Length 44 mm. f/5.6

Spacing and Clear Lens N Radii, Thicknesses, Apperture Element mm. mm.Dia. mm. I l.49l R +6,993 l0.922

S,=0.375 [I 1.567 R,-95.25 8.432

T,=0.96K lid-6.544 8.686 S,=l.665 III [.567 R +l4.943 7.620

T,=i.s00 R.7l.654 7.823 S =0.984 to Stop The data given are for anobjective lens system having an equivalent focal length of 44 mm. Tomake a triplet objective lens system having a different focal length itis only necessary to scale all dimensions in the same proportion.

An objective constructed in accordance with the foregoing data has amaximum relative aperture of f/5.6 and a back focal length of 35.37 mm.With this objective the stop or diaphragm diameter may be on the orderof 6.274 mm., and the position of the stop of diaphragm 14 may be 35.2mm. from the film plane with the lens system focused at infinity.

Of particular importance is the fact that with the specified indices ofrefraction Nd, all lens elements of the objective lens system may beconstructed of plastic materials such as acrylic materials. For example,lens element I may be formed of methyl methacrylate and lens elements IIand III may be formed of polyacrylonitrile. These materials provideadequate optical quality and they permit the system to be readilyfabricated at lower cost and with high volume production techniques. Allof the lens elements may be molded from these plastic materials. I

The invention has been described in detail with particular reference tothe preferred embodiment thereof, and it is intended that thisdescription be interpreted as being exemplary and illustrative and notin limiting sense.

it is to be understood that the present disclosure has been made only byway of example and that many additional modifications and changes invarious details may be resorted to without departing from the invention.

What is claimed is:

l. A triplet objective lens system consisting of two outer positive lenselements and an intermediate negative lens element, said lens elementshaving substantially the following characteristics and spacerelationships as related to a focal length of 44 millimeters, eachnumbered from front to rear, and wherein N,, is the index of refraction,R is the radius of curvature of the lens surface, T is the axialthickness of the lens element, and S is the axial air spacing betweenthe lens elements:

Spacing and Lens Radii Thicknesses,

d Element mm. mm. l L49] R,+6,993

2. The triplet objective lens system of claim 1 wherein diameters of theclear apertures for each of the lens surfaces are as follows:

Lens Clear Aperture Surface in mm. R 10.922 R, 8.737 R, 8.432 R. 8.686R, T620 R. 7'82 ment (lll).

UNTTED STATES PATENT OFFICE CERTIFICATE @F CURRECTION Patent No. 3 B 734,237 Dated Janna, 1974 Inventofls) I DAVID S. GREY It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

ColunmhZ; line 37 "appertu re" should be ----faper.t ure col-111M 2 l irie' 39 and Claim 1 "R 6,993",,

vshould be R 6.993

Column 2,' line 45 "T 1,800 should appear in.4th column of Table]:

Signed end sealed this 19thday of November 1974.

(SEAL) Attest: McCOY M. GIBSON JR.- v v c, MARSHALL DANN AttestingOfficer 1 v v 1 Commissioner of Patents us'coMM-oc man-Poo r I l 9 "-5.GOVRNHIIIT PRINTING OFFICE il O-lllll

1. A triplet objective lens system consisting of two outer positive lenselements and an intermediate negative lens element, said lens elementshaving substantially the following characteristics and spAcerelationships as related to a focal length of 44 millimeters, eachnumbered from front to rear, and wherein Nd is the index of refraction,R is the radius of curvature of the lens surface, T is the axialthickness of the lens element, and S is the axial air spacing betweenthe lens elements: Spacing and Lens Radii Thicknesses, Nd Element mm.mm. I 1.491 R1+6,993 T1 2.655 R2+42.55 S1 0.375 II 1.567 R3-95.25 T20.968 R4+6.544 S2 1.665 III 1.567 R5+14.943 T3 1.800 R6-71.654
 2. Thetriplet objective lens system of claim 1 wherein diameters of the clearapertures for each of the lens surfaces are as follows: Lens ClearAperture Surface in mm. R1 10.922 R2 8.737 R3 8.432 R4 8.686 R5 7.620 R67.823
 3. The triplet objective lens system of claim 1 wherein said lenselements are constructed of acrylics.
 4. The triplet objective lenssystem of claim 3 wherein lens elements II and III are constructed ofpolyacrylonitrile.
 5. The triplet objective lens system of claim 4wherein lens element I is constructed of methyl methacrylate.
 6. Thetriplet objective lens system of claim 1 wherein a diaphragm is providedand is spaced 0.984 mm. rearwardly of the rearward condensing lenselement (III).